In a typical fuel control system of a gas turbine engine, the fuel control system adjusts the fuel flow to meet the operator's demands. The objective of the fuel control system is to be responsive to the operator's demand while at the same time protecting against stall, surge, or inefficient operation of the engine. The fuel control system monitors certain parameters of the engine and inputs the parameters into a variety of fuel flow schedules to optimize the operation of the engine.
One of the parameters which is monitored by the fuel control system is the fuel metering valve position. The fuel metering valve meters the rate of fuel flow to the engine. Monitoring the fuel metering valve position provides a feedback to the control system to determine changes which are required to meet the operator's demand. The fuel metering valve position is also necessary to provide updates as to hardware characteristics to ensure proper biasing of the fuel metering valve. The fuel metering valve is positioned by a torque motor. The torque motor has a null bias which ensures that the failsafe position of the metering valve corresponds to a minimum flow condition.
A failure in the monitoring of the fuel metering valve position generally leads to a failsafe positioning of the fuel metering valve. For safety reasons, the failsafe position is a minimum flow position. This position may result in an engine shut down. The frequency of engine shut downs during flight is referred to as the in-flight shut down (IFSD) rate. It is desirable to minimize the IFSD rate. One means to minimize the IFSD rate is to build redundancy into the fuel control system. While providing backup controls and inputs is one solution, it increases the cost, weight and complexity of the fuel control system.